feat(authz): operator signed-op verifier + durable nonce store (slice 2, v0.2.0)

internal/authz: production form of the Phase-4 SSHSIG signing primitive.

- Verifier.New/Verify with the LOCKED pipeline (namespace → allow-list by key
  material → crypto over RAW bytes → target → time → nonce LAST); each post-crypto
  stage rejects even with a valid sig; an invalid sig never burns a nonce.
- SSHSIG framing via x/crypto/ssh (no hand-rolled crypto); key-type-agnostic
  (ed25519 / sk-ssh-ed25519 / rsa / ecdsa via pub.Verify). Fixed namespace
  felhom-op-v1. Typed errors. OpBlob (fixed host_id/guest_id tags) + VerifiedOp.
- NonceStore: MemoryNonceStore + durable crash-safe FileNonceStore (fsync'd append
  log, replay-on-open, compaction, expiry-only pruning; survives restart).
- config.AuthzConfig (nonce path + pinned operational/recovery signer keys).
- Tests (14): real ssh-keygen fixture, per-stage rejection, nonce-not-burned,
  replay, persistence-across-restart, synthetic sk, byte-exactness.

Dep: golang.org/x/crypto v0.52.0 (declares go 1.25 — the Phase-4 doc's "Go 1.24.4 /
x/crypto v0.52.0" pairing doesn't build; build server upgraded to go1.26.0,
backward-compatible). Version 0.1.0 -> 0.2.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/authz/signer_test.go

@@ -0,0 +1,36 @@
+package authz
+
+import (
+	"os"
+	"testing"
+)
+
+func TestNewAllowedSigner(t *testing.T) {
+	line, err := os.ReadFile("testdata/operator.pub")
+	if err != nil {
+		t.Fatal(err)
+	}
+	s, err := NewAllowedSigner("felhom-op-1", RoleOperational, string(line))
+	if err != nil {
+		t.Fatalf("NewAllowedSigner: %v", err)
+	}
+	if s.KeyID != "felhom-op-1" || s.Role != RoleOperational || s.PublicKey == nil {
+		t.Errorf("signer = %+v", s)
+	}
+	if s.PublicKey.Type() != "ssh-ed25519" {
+		t.Errorf("key type = %q", s.PublicKey.Type())
+	}
+}
+
+func TestNewAllowedSigner_BadRole(t *testing.T) {
+	line, _ := os.ReadFile("testdata/operator.pub")
+	if _, err := NewAllowedSigner("k", "bogus", string(line)); err == nil {
+		t.Fatal("invalid role should error")
+	}
+}
+
+func TestNewAllowedSigner_BadLine(t *testing.T) {
+	if _, err := NewAllowedSigner("k", RoleOperational, "not a key"); err == nil {
+		t.Fatal("malformed key line should error")
+	}
+}